void fitSTR(){
  const double t[] = {0., 4.750, 9.949, 13.069, 23.469, 35.949, 44.269, 60.908, 76.508, 112.907, 147.227, 179.466, 200.};
  const double r[] = {0., 0.404, 0.841, 1.049,  1.570,  2.236,  2.756,  3.401,  4.150,  4.671,   4.900,   5.,      5.};
  static const int n = sizeof(t)/sizeof(double);

  TGraph *g = new TGraph(n, t, r);
  g->SetNameTitle("rt", "space-time relation");
  g->SetMarkerColor(2);
  g->SetMarkerStyle(20);
  TCanvas *c = new TCanvas("c", "calibration of Z", 800, 600);
  TF1 *rt = new TF1("rt", "pol4");
  g->Draw();
  g->Fit(rt);
  cout << "r(0) = " << rt->Eval(0.) << endl;
  TFile *f = new TFile("rt.root", "recreate");
  g->Write();
} // end function fitZ
